The invention relates in particular to double-action antenna structures suitable for mobile stations, in which structures one component is a retractable whip element.
In the field of portable radio equipment, mobile stations in particular, the manufacture of antennas has become very demanding. As new frequency bands are introduced, an antenna often has to function in two or more frequency bands. When the devices are small, the antenna, too, must be small; preferably it is placed inside the covering of the apparatus, thus avoiding an impractical protrusion. Understandably, however, the radiation characteristics of an internal antenna are somewhat poorer that those of an external antenna. Moreover, an internal antenna is more sensitive to the effect of the hand of the user, for example. These disadvantages can be reduced using a double-action antenna so that a movable antenna element belonging to the structure can be pulled partly out when necessary in order to improve the quality of the connection.
A retractable whip element is well known as such. If the antenna structure additionally comprises a second radiating element, it is usually an element outside the covering of the apparatus, considerably shorter than the whip element. Such a double-action antenna, which in one operating mode is located completely inside the covering of the apparatus, is disclosed in an earlier patent application F1991359 by the same applicant. The structure is depicted in FIG. 1. It comprises a ground plane 110, radiating planar element 120, feed conductor 102 and a short-circuit conductor 103, which constitute a PIFA (Planar Inverted F Antenna) type portion of the whole antenna, located inside the covering of the radio apparatus. The planar element 120 has a slot 125 in it, which is shaped such that the resonance frequency of the planar antenna is as desired. The structure further includes a whip element 130, at the lower end of which there is a connecting part 131. When the whip is in its lower position, it has no significant coupling with the PIFA parts. When the whip is in its upper position, the connecting part 131 is in galvanic contact with the planar element 120 on both sides of the slot 125 so that the slot becomes short-circuited. Short-circuiting the slot considerably increases the resonance frequency of the planar antenna, whereby the planar antenna will not function as an antenna in the operating frequency band when the whip is in the pulled-out position. The whip element is so dimensioned that it will function as a monopole antenna in the same operating frequency band, thereby replacing the internal planar antenna. The task of the planar element 120 is then to function as a part in the feed line of the whip and as an impedance-matching element of the whip. The PIFA may also be arranged so as to have two frequencies so that in its upper position the whip element changes e.g. the lower resonance frequency of the PIFA in such a manner that only the pulled-out whip functions as the radiating element at the lower operating frequency. Then the conductive plane of the PIFA functions as the radiating element at the upper operating frequency. Alternatively, the pulled-out whip element just enhances the operation of the antenna at the lower operating frequency without changing the resonance frequency of the PIFA.
It is an object of the invention to provide a double-action antenna in a novel and more advantageous manner than in known structures. The antenna structure according to the invention is characterized by what is specified in the independent claim 1. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: An antenna structure comprises an antenna located inside the covering of a mobile station, a switch and a whip element movable in relation to the former two. The internal antenna comprises two elements one of which is connected to the feed conductor of the whole antenna structure and the other to the signal ground through a short-circuit conductor. When the whip element is retracted, said switch galvanically connects the elements of the internal antenna to one another. Then, only the internal antenna is in use and the whip has no practical significance. When the whip element is pulled out, its lower end disconnects, by means of the switch, the elements of the internal antenna from one another, and the whip element itself is connected in series with that element at one end of which the feed conductor of the antenna structure is joined. Thus the series connection of the whip and the element in question functions as a radiator, and the shorted element of the internal antenna has no practical significance. In addition, the internal element of the series connection provides for the matching of the whip element.
An advantage of the invention is that in the structure according to the invention the length of the whip element can be chosen relatively freely. This is due to the fact that by means of the internal antenna element connected in series with the whip element the electrical length of the structure can be made e.g. a quarter of the wavelength or three quarters of the wavelength. Another advantage of the invention is, in accordance with the above, that no separate mechanical parts or components are needed for the matching of the whip element. A further advantage of the invention is that the structure according to the invention is relatively simple and inexpensive to manufacture.